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AromaticAromaticity is a chemical property in which a conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibit a stabilization stronger than would be expected by the stabilization of conjugation alone. It can also be considered a manifestation of cyclic delocalization and of resonance . This is usually considered to be because electrons are free to cycle around circular arrangements of atoms, which are alternately single- and double-bonded to one another. These bonds may be seen as a hybrid of a single bond and a double bond, each bond in the ring identical to every other. This commonly-seen model of aromatic rings was developed by Kekulé. The model for benzene consists of two resonance forms, which corresponds to the double and single bonds' switching positions. Benzene is a more stable molecule than would be expected of cyclohexatriene, which is a theoretical molecule. By convention, the double-headed arrow indicates that the two structures are simply hypothetical, since neither is an accurate representation of the actual compound. The actual molecule is best represented by a hybrid (average) of these structures, which can be seen at right. A C=C bond is shorter than a C−C bond, but benzene is perfectly hexagonal--all six carbon-carbon bonds have the same length, intermediate between that of a single and that of a double bond. A better representation is that of the circular π bond (Armstrong's inner cycle), in which the electron density is evenly distributed through a π bond above and below the ring. This model more correctly represents the location of electron density within the aromatic ring. The single bonds are formed with electrons in line between the carbon nuclei--these are called sigma bonds. Double bonds consist of a sigma bond and another bond--a π bond. The π-bonds are formed from overlap of atomic p-orbitals above and below the plane of the ring. The following diagram shows the positions of these p-orbitals: Since they are out of the plane of the atoms, these orbitals can interact with each other freely, and become delocalised. This means that, instead of being tied to one atom of carbon, each electron is shared by all six in the ring. Thus, there are not enough electrons to form double bonds on all the carbon atoms, but the "extra" electrons strengthen all of the bonds on the ring equally. The resulting molecular orbital has π symmetry. The attribution of this exceptional stability is conventionally to Sir Robert Robinson, who was apparently the first (in 1925) to coin the term aromatic sextet as a group of six electrons that resists disruption. In fact, this concept can be traced further back, via Ernest Crocker in 1922, to Henry Edward Armstrong, who in 1890, in an article entitled The structure of cycloid hydrocarbons, wrote the (six) centric affinities act within a cycle...benzene may be represented by a double ring (sic) ... and when an additive compound is formed, the inner cycle of affinity suffers disruption, the contiguous carbon-atoms to which nothing has been attached of necessity acquire the ethylenic condition . Check out the following recipes that are tagged "Aromatic":
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